The present invention relates to a method and system for balancing traffic load between devices and base stations, and in particular, to a method and system for balancing traffic load such that communication traffic is well distributed among base stations by allowing base stations to represent a station load condition to devices for prospective data communication a manner which does not adversely impact base station performance.
The success of wireless communications has increased demand for new types of wireless devices as well as for an increasing quantity of these devices. However, as success has grown, so has the burden placed on the communication infrastructures which support the wireless devices. The communication infrastructure includes base stations, used to communicate with the wireless devices, arranged in a network with access being provided to external services, for example, Internet access. The infrastructure exists in the form of increasing base station and antenna densities, as well as the increasing processing loads placed on base station communication equipment. This is particularly the case because the wireless devices are mobile, thus requiring that communication be handed off from one base station to another.
During movement requiring a switch in communication from one base station to another, the handoff protocols, such as xe2x80x9csoft handoffxe2x80x9d protocols in a code division multiple access environment, typically place an extra processing load on the base station. In addition, a base station executing at processing capacity may not be able to accommodate additional wireless devices.
It is desired, therefore, to have a system which allows communication between a wireless device and a base station to be handed to another base station which can accommodate the additional processing and spectrum load posed by the wireless device.
In orthogonal frequency division multiplexing (OFDM), an OFDM symbol is comprised of many subcarriers. These subcarriers carry both control and data information. Typically, a dedicated set of subcarriers is used as pilots and are transmitted by the base station with maximal power. Referring to FIG. 1, a group of subcarriers 10 is shown in which data subcarriers 12 are transmitted at the same power level as pilot subcarriers 14.
Of course, based on factors such as the distance from the wireless devices to the base station, multipath fading and shadowing, the received power at the wireless devices will vary from one another. Because a communication channel can be frequency selective, i.e., have different fading rates among subcarriers, the received power at a particular device will also vary between the received subcarriers. Given these variations, the communication data rate which can be supported between the wireless device and the base station is proportional to the carrier-to-interference (C/I) ratio as measured by the wireless device.
Typically, wireless terminals are arranged to initiate communication with, or handoff to, the base station having the greatest C/I ratio. This arrangement is known as site selection diversity transmission (SSDT). Current methods of SSDT, for example those proposed and included in wireless communication standards such as the 3rd Generation Partnership Project (3GPP) standard and the High Data Rate (HDR) proposed standard, take into account the received signal strength; but as discussed above, the methods fail to identify or take into account the processing and wireless device quantity loads (hereinafter together referred to as xe2x80x9cloadxe2x80x9d) of the base stations. These arrangements therefore do nothing to balance the communication load among base stations. As such, even though a wireless device might be receiving the best signal from a particular base station, that base station might be too loaded to service the added wireless device. The result is a dropped call or a communication session perceived by the user of the wireless device as poor.
Further, arrangements in which the base station must proactively reject communication with a wireless device waste base station processing resources, further loading a potentially overloaded base station. In other words, systems which rely on the base station to make the decision as to whether to accept or reject a communication request made by a wireless device disadvantageously worsen the loading problem. Also, this arrangement adds unnecessary communication delay because the wireless terminal must wait to receive an indication as to whether the communication request is accepted or rejected.
It is therefore desirable to have a system and method which links site selection diversity to the load of the base station in a manner which does not adversely effect the coverage area of the base station or the data throughput of the base station, particularly in an OFDM environment.
The present invention provides a method and system for facilitating load-balanced communication between communication devices and base stations, particularly in an OFDM wireless communication environment. The load balancing is achieved without adversely impacting system performance and without reducing the coverage area of each base station. The load balancing is preferably accomplished by minimizing the amount of CPU resources consumed during communication initialization and/or communication handoff from one base station to another.
Further, the present invention advantageously employs a modulation/coding scheme which is arranged to facilitate data communication between a device and a base station in a load-balanced environment in a manner which optimizes channel utilization.
As one aspect of the invention, a method for selecting a station, in a communication system having a plurality of stations, for communication with a device based on a station load condition of the respective stations, is provided in which the station load condition for each of the plurality of stations is determined. The station transmit power level for at least a part of a transmitted signal in each of the plurality of stations is determined in accordance with the respective determined station load condition. The transmitted signal from at least one of the plurality of stations is received. A station for communication is selected. The station selection is made by the device based at least in part on the received power level of the adjusted portion of the transmitted signal.
As still another aspect, the present invention provides a substantially load-balanced communication system having at least one station and at least one communication device. Each station determines its station load condition and adjusts its station transmit power level for at least a part of a transmitted signal in accordance with the respective determined station load condition. The station transmit power level is adjusted to substantially balance the station load conditions among the stations. Each device receives the transmitted signal from the at least one station and selects a station for communication. The station selection is made by the device based at least in part on the received power level of the adjusted portion of the transmitted signal.
According to another aspect, the communicating system is an OFDM system.
As still yet another aspect, the present invention provides a base station for communication with a device, in which the base station has a transmitter transmitting a signal to the device. A central processing unit controls the transmitter by determining a base station load condition and adjusting the station transmit power level for at least a part of the signal. The station transmit power is adjusted in accordance with the determined station load condition.
Each base station is preferably arranged to determine its load based on one or more load factors and manifest that load condition in the form of the above-described reduced carrier power output. The base station takes this adjustment into account when determining the modulation and coding scheme to employ for data communication with a device.
According to another aspect, the present invention provides a device for communication with one or more base stations, in which the device has a receiver receiving a signal from at least one of the base stations. A central processing unit is in operative communication with the receiver and determines a carrier to interference power ratio for each of the received signals and selects a base station for data communication based on the determined carrier to interference power ratios. The carrier to interference ratios are based, at least in part, on a loading condition of the base station transmitting the corresponding signal.